High-resolution spectroscopy in TR 37: Gas accretion evolution in evolved dusty disks

摘要

Using the Hectochelle multifiber spectrograph, we have obtained high-resolution (R similar to 34,000) spectra in the H alpha region for a large number of stars in the 4 Myr old cluster Tr 37, containing 146 previously known members and 26 newly identified ones. We present the H alpha line profiles of all members, compare them to our IR observations of dusty disks (Two Micron All Sky Survey JHK + IRAC + MIPS 24 mu m), use the radial velocities as a membership criterion, and calculate the rotational velocities. We find a good correlation between the accretion-broadened profiles and the presence of protoplanetary disks, noting that a small fraction of the accreting stars presents broad profiles with H alpha equivalent widths smaller than the canonical limit separating classical T Tauri stars (CTTSs) and weak-lined T Tauri stars (WTTSs). The number of strong accretors appears to be lower than in younger regions, and a large number of CTTSs have very small accretion rates (M <= 10(-9) M-circle dot yr(-1)). Taking into account that the spectral energy distributions are consistent with dust evolution (grain growth/settling) in the innermost disk, this suggests a parallel evolution of the dusty and gaseous components. We also observe that about half of the "transition objects'' (stars with no IR excesses at lambda <= 6 mu m) do not show any signs of active accretion, whereas the other half is accreting with accretion rates <= 10(-9) M-circle dot yr(-1). These zero or very low accretion rates reveal important gas evolution and/or gas depletion in the innermost disk, which could be related to grain growth up to planetesimal or even planet sizes. Finally, we examine the rotational velocities of accreting and nonaccreting stars, finding no significant differences that could indicate disk locking at these ages.